Recently, electron excited fluorescent display devices such as cathode ray tubes and low-velocity electron excited fluorescent display devices are used for various purposes, and need exists for phosphors exhibiting linear, sublinear or superlinear excitation energy-emission luminance characteristics over wide range of the excitation energy.
The electron excited fluorescent display devices of this type include cathode ray tubes such as a multi-color cathode ray tube, a projection cathode ray tube and a high-luminance color cathode ray tube, multi-color low-velocity electron excited fluorescent display devices, and the like. These electron excited fluorescent display devices are provided with fluorescent screens comprising several kinds of phosphors mixed or combined together, and required to exhibit light emission characteristics corresponding to the characteristics of the phosphors. Particularly, strong need exists for a red emitting phosphor exhibiting sublinear excitation energy-emission luminance characteristics. For example, a multi-color cathode ray tube recently employed in terminal display units of a computer system, display units of an aircraft control system, and the like comprises a fluorescent screen constituted by two kinds of phosphors emitting light of colors different from each other; i.e. a phosphor exhibiting superlinear excitation energy-emission luminance characteristics when energy of excitation electron beam increases, and a phosphor exhibiting sublinear excitation energy-emission luminance characteristics when energy of the excitation electron beam increases. In the multi-color cathode ray tube, the color of light emitted from the fluorescent screen is changed by changing the energy of the excitation electron beam, thereby multi-color display is effected.
The aforesaid multi-color cathode ray tube is classified into two types according to the methods of changing the energy of the excitation electron beam. In one type thereof, the energy of the excitation electron beam is changed by changing the acceleration potential. In the other type, the energy of the excitation electron beam is changed by changing the current density. The former type is called potential modulation type multi-color cathode ray tube, and the latter type is called current modulation type multi-color cathode ray tube. The potential modulation type multi-color cathode ray tube is described, for example, in "Nikkei Electronics", July 2, 1973, pages 106 to 117, and the current modulation type multi-color cathode ray tube is described in Japanese Patent Publication No. 52(1977)-5225.
The current modulation type multi-color cathode ray tube is advantageous over the potential modulation type multi-color cathode ray tube in that the constructions of the electron gun and the electron gun control circuit, and the like are markedly simple. In spite of this advantage, the current modulation type multi-color cathode ray tube is not so widely employed for practical use as the potential modulation type multi-color cathode ray tube. This is because there are known few phosphors suitable for use in the current modulation type multi-color cathode ray tube, i.e. the phosphors exhibiting sufficiently superlinear or sublinear current density-emission luminance characteristics when the current density of the excitation electron beam increases.
As the phosphors exhibiting superlinear current density-emission luminance characteristics (hereinafter referred to as superlinear characteristics) when the current density of the excitation electron beam increases, there have heretofore been known a copper and aluminium activated zinc sulfide green emitting phosphor (ZnS:Cu,Al) containing at least one metal selected from the group consisting of iron, cobalt and nickel in an appropriate amount, and a silver activated zinc cadmium sulfide green-blue to red emitting phosphor [(Zn,Cd)S:Ag] containing at least one metal selected from the group consising of iron, cobalt and nickel in an appropriate amount (as is well known, this phosphor emits green-blue to red light according to a change in the molar ratio of ZnS to CdS). Further, as the phosphor exhibiting sublinear current density-emission luminance characteristics (hereinafter referred to as sublinear characteristics) when the current density of the excitation electron beam increases, there is known a manganese activated zinc silicate green emitting phosphor (Zn.sub.2 SiO.sub.4 :Mn).
In general, in order to widen the range of color displayed on a multi-color cathode ray tube, the fluorescent screen is constituted by:
(i) a combination of a red emitting phosphor exhibiting superlinear excitation energy-emission luminance characteristics with a green emitting phosphor exhibiting sublinear excitation energy-emission luminance characteristics, or
(ii) a combination of a green-blue to green emitting phosphor exhibiting superlinear excitation energy-emission luminance characteristics with a red emitting phosphor exhibiting sublinear excitation energy-emission luminance characteristics. In view of the above, it is contemplated to constitute the fluorescent screen of a current modulation multi-color cathode ray tube by use of a combination of the aforesaid (Zn,Cd)S:Ag red emitting phosphor containing at least one metal selected from the group of iron, cobalt and nickel (exhibiting the superlinear characteristics) with the aforesaid Zn.sub.2 SiO.sub.4 :Mn green emitting phosphor (exhibiting the sublinear characteristics). On the other hand, however, there has not heretofore been known any red emitting phosphor exhibiting good sublinear characteristics suitable for use in combination with the aforesaid ZnS:Cu,Al green emitting phosphor containing at least one metal selected from the group consisting of iron, cobalt and nickel or with the aforesaid (Zn,Cd)S:Ag green-blue to green emitting phosphor containing at least one metal selected from the group consisting of iron, cobalt and nickel (both phosphors exhibit the superlinear characteristics).
Also in electron excited fluorescent display devices other than the above-described current modulation type multi-color cathode ray tube, since there is not known any yellow to red emitting phosphor exhibiting good sublinear characteristics matching the current saturation characteristics of another phosphor employed together, very real problems such as color drift occurring at a high current arise.
Accordingly, there has been strong need for a yellow to red emitting phosphor exhibiting the sublinear characteristics, and a multi-color cathode ray tube and a multi-color low-velocity electron excited fluorescent display device exhibiting a wide reproduced color range, or a high luminance color cathode ray tube free of color drift at a high current.
The primary object of the present invention is to provide a yellow to red emitting phosphor exhibiting good sublinear characteristics.
Another object of the present invention is to provide an electron excited fluorescent display device having a wide reproduced color range by use of the yellow to red emitting phosphor exhibiting good sublinear characteristics.
The specific object of the present invention is to provide an electron excited fluorescent display device free of color drift at a high current by virtue of the use of the yellow to red emitting phosphor exhibiting good sublinear characteristics.
It is already known that an europium activated rare earth oxysulfide phosphor represented by the general formula: EQU (Ln,Eu).sub.2 O.sub.2 S
in which Ln is at least one of yttrium, gadolinium, lanthanum and lutetium, emits yellow to red light of high luminance under electron excitation according to the level of the Eu activator amount contained in the phosphor. For example, a Y.sub.2 O.sub.2 S:Eu phosphor which is one of the phosphors of this type and contains a relatively large amount of the Eu activator is presently employed in the practical use as a red emitting component phosphor of a color television cathode ray tube.
It is known that, as shown in FIG. 1, the (Ln,Eu).sub.2 O.sub.2 S phosphor exhibits very slight sublinear characteristics. In view of the ability of the (Ln,Eu).sub.2 O.sub.2 S phosphor to emit light of high luminance on the practical use level, the inventors of the present invention carried out various investigations to improve sublinearity exhibited by the (Ln,Eu).sub.2 O.sub.2 S phosphor for the purpose of accomplishing the aforesaid objects of the present invention. As a result of the investigations, it was found that, when cerium (Ce) serving as a coactivator employed together with Eu is used in an amount within a predetermined range, it is possible to improve sublinearity of the current density-emission luminance relationship (hereinafter referred to as sublinearity) exhibited by the (Ln,Eu).sub.2 O.sub.2 S phosphor. Further, it was found that the aforesaid sublinearity is markedly improved when the amount of the Ce coactivator is within a particularly defined range falling within the aforesaid predetermined range. Based on these findings, the present invention has been completed.